1. Field of the Invention
This invention relates to a radiation shielding grid for use with a radiation detection panel comprising a plurality of spaced discreet radiation sensing elements, and more particularly to a method for designing such grid to eliminate Moirxc3xa9 patterns and to the resulting grid.
2. Description of Related Art
Direct radiographic imaging using panels comprising a two dimensional array of minute sensors to capture a radiation generated image is well known in the art. The radiation is imagewise modulated as it passes through an object having varying radiation absorption areas. Information representing an image is captured as a charge distribution stored in a plurality of charge storage capacitors in individual sensors arrayed in a two dimensional matrix.
X-ray images are decreased in contrast by X-rays scattered from objects being imaged. Anti-scatter grids have long been used (Gustov Bucky, U.S. Pat. No. 1,164,987 issued 1915) to absorb the scattered X-rays while passing the primary X-rays. Whenever the X-ray detection panel resolution is comparable or higher than the spacing of the grid, an image artifact from the grid may be seen. Bucky also taught moving the anti-scatter grid to eliminate that image artifact by blurring the image of the anti-scatter grid (but not of the object, of course). The anti-scatter grid may be linear or crossed. Bucky furthermore taught a focused anti-scatter grid.
Improvements to the construction of anti-scatter grids have reduced the need to move the grid, thereby simplifying the apparatus and timing between the anti-scatter grid motion and X-ray generator. However, Moirxc3xa9 pattern artifacts can be introduced when films from such apparatus are digitized. Image intensifiers for fluoroscopy can also produce Moirxc3xa9 pattern artifacts. It is known and recommended to align the bars of a linear anti-scatter grid perpendicular to the direction of scan (The Essential Physics of Medical Imaging, Jerrold T Bushberg, J. Anthony Seibert, Edwin M. Leidholdt,Jr., and John M. Boone. c1994 Williams and Wilkins, Baltimore, pg. 162 ff.).
When the X-ray detection panel is composed of a two dimensional array of picture elements or X-ray sensors, as opposed to film or raster scanned screens, the beat between the spatial frequency of the sensitive areas and that of the anti-scatter grid gives rise to an interference pattern having a low spatial frequency, i.e. a Moirxc3xa9 pattern. U.S. Pat. No. 5,666,395 to Tsukamoto et al. teaches Moirxc3xa9 pattern prevention with a static linear grid having a grid pitch that is an integer fraction of the sensitive area pitch.
Two cases are discussed in the aforementioned patent. In the first, the sensors are positioned in the array so that there is no dead space between sensor elements. In this instance, the grid pitch is made equal to an integer fraction of the sensor pitch, the distance between adjacent sensor centers.
In the second case where the sensors are separated by dead spaces, i.e. interstitial spaces which are insensitive to radiation detection, the grid pitch is made to correspond to the sensor pitch and is held in a steady positional relation to the detection panel such that the grid elements are substantially centered over the interstitial spaces.
A problem with the above proposed solutions is that it is difficult to construct a radiation detection panel having no interstitial spaces between adjacent sensor elements. When there are interstitial spaces present, maintaining the anti-scatter grid in a fixed position relative to the radiation sensor array is often impractical.
There is thus still need for a grid that will shield an X-ray radiation sensor array comprised of discreet non contiguous elements from incident scattered radiation, which does not require a fixed positioning relative to the radiation detection panel, or moving during exposure to avoid creating Moirxc3xa9 patterns.
According to this invention there is provided a scattered radiation shielding grid comprising a radiation absorbing material representing a pattern corresponding to a combined motif of a plurality of tiled prototiles, each prototile comprising a width W(p), a length and a motif solely within the prototile, wherein the prototile width W(p)=W/I where W is a width of a radiation sensitive area of a radiation sensor of a radiation detection panel comprising a plurality of equal size radiation sensors separated by interstitial spaces having a width D, over which said grid is positioned, and where I is an integer.
In accordance with this invention, there is also provided a scattered radiation shielding grid comprising a radiation absorbing material, and a radiation detection panel over which said grid is positioned, the radiation panel comprising a plurality of equal size radiation sensors having a radiation sensitive area width W, separated by radiation insensitive interstitial spaces having a width D, and wherein the grid absorbing material forms a pattern representing a combined motif of a tiled plurality of substantially identical prototiles, each prototile comprising:
(a) a width W(p)=W/I, wherein I is an integer;
(b) a length; and
(c) a motif contained solely within the prototile.
Still in accordance with this invention, the detection panel may further comprise a gain correction circuit associated with said detection panel, in which case W(p)=W/(Ixc2x10.051) and W(p)xe2x89xa0W+D.
When the scattered radiation grid and detection panel according to this invention is used with a radiation source, and the grid is positioned between the panel and the radiation source at a fixed, known distance from said panel, the prototile width W(p) is a projected prototile width on said panel.
Still according to the present invention there is provided a method for designing a pattern for the absorption material to be used to form a scattered radiation shielding grid for a radiation detection panel comprising an array of a plurality of sensors each sensor having a radiation sensitive area, the sensors arrayed so that each radiation sensitive area is separated by each adjacent radiation sensitive area by an interstitial space having a width D, the method comprising:
a) determining a sensor width W corresponding to a width of a radiation sensitive area of the sensor
b) creating a prototile having a width W(p)=W/I wherein I is an integer;
c) producing within said prototile a motif and
d) tiling a plurality of said prototiles to produce the pattern, said pattern consisting of the combined motif of the tiled prototiles.
Still according to the present invention there is provided a method for manufacturing a scattered radiation shielding grid comprising a pattern of radiation absorbing material for a radiation detection panel comprising an array of a plurality of sensors, each sensor having a radiation sensitive area having a width W and a length, the sensors arrayed so that each radiation sensitive area is separated by each adjacent radiation sensitive area by an interstitial space having a width D, the method comprising:
a) determining a senor width W corresponding to the width of the radiation sensitive area of the sensor
b) creating a prototile having a width W(p)=W/I wherein I is an integer;
c) producing within said prototile a motif;
d) tiling a plurality of said prototiles to produce a pattern consisting of the combined motif of the tiled prototiles;
e) forming said radiation absorbing material in said grid in the shape of said combined motif.
Also in accordance with this invention there is provided a method for forming a radiogram with an exposure system comprising radiation source, and a radiation detection panel, wherein the radiation detection panel comprises an array of a plurality of sensors each having a radiation sensitive area having a width W and a length, the sensors arrayed so that each radiation sensitive area is separated by each adjacent radiation sensitive area by an interstitial space having a width D, the method comprising:
positioning between said radiation source and said panel a grid comprising a radiation absorbing material formed in a pattern representing a combined motif of a plurality of substantially identical tiled prototiles, each prototile comprising a width W(p), a length and said motif, said motif contained solely within the prototile, wherein the prototile width W(p) =W/I where I is an integer.